EMX : RTC battery lifespan

Hello all,

I have designed a board for EMX, and I wonder what battery should I use (connect to Vbat)to allow the board to keep the correct date several years :

-According to LPC2478 documentation and to amperemeter measure, the current is approx. 20 microAmps, so for example, if I use a small CR1216 25mAh coin cell, the RTC should fail in less than 2 months : (25000 / 20 ) = 1250 hours = 52 days < 2 months

-So I am currently looking for biggest batteries for RTC : (even biggest coin cells are not enough) Those 2500 mAh 3.6V seems OK for nearly 15 years : http://www.farnell.com/datasheets/1443075.pdf

I am just surprised that a coin cell is not enough, because coin cells are used in old computers, and I guess that they keep the correct date several yearsâ€¦ Do I miss something?

If you have a device that get power every few days, a super cap maybe an option.

You counting total offline time. But how often and for how long is your computer really off-line?

My computation shows that a CR2032 shoould last a year (about 400 days) a CR2450 should last more than two years. For sure this is not enough for high end productsâ€¦ May be I am wrong but even with 2 superc caps of 5F (2.5V) Data can be lost after 3 months without switching on the deviceâ€¦ I do not see any good solutionâ€¦

battery and charging module has a cost, and takes space. i donâ€™t say thereâ€™s no solution but for people that have already pcb in productionâ€¦ itâ€™s too late

have a look at

http://www.ghielectronics.com/community/forum/topic?id=11683

they calculate a longer time

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[quote]have a look at
http://www.ghielectronics.com/community/forum/topic?id=11683
they calculate a longer time[/quote]
->Yes, but they calculate with a CR2032 coin cell battery, with is 240mAh, and I am only using a CR1216 (40mAh) So if I summarize the RTC consumes 20microAmps and so:

The expected lifespan of RTC counting while system is powered off are:

-With a CR1216 coin cell (typ : 40mAh) : 3 months
-With a CR2032 coin cell (typ : 210mAh) : 18 months
-With biggest batteries 3.6v / 1200mAh : 10 years (perfect for systems wich should live a long time, with short times turned on)

I will try this third solution! Indeed, I hope that 3.6V is not to much for Vbat inputâ€¦ I will take a look on datasheetâ€¦ in the other case, I have to find a 3v3 battery wich is more than 1000mAh!

What I still not understand is : When EMX is powered, is RTC always powered from Vbat pin or from EMX main 3v3? i.e. is really the battery connected on Vbat NOT discharging when EMX is powered on?

According to the datasheet,

[quote]The VBAT pin supplies power only to the RTC and the Battery RAM. These two functions
require a minimum of power to operate, which can be supplied by an external battery.
When the CPU and the rest of chip functions are stopped and power removed, the RTC
can supply an alarm output that can be used by external hardware to restore chip power
and resume operation.[/quote]

Andâ€¦ a max of 4v6 so even a fully charged Li-Ion 18650 with 3000mAh would be fineâ€¦ EDIT: thatâ€™s Absolute max of 4.6v (from Section 8 of datasheet) and 3.6v listed as max in the static characteristics section (section 10)

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